This invention relates generally to differential relays which are used for protection of power system apparatus, and more specifically concerns such a differential relay which is capable of responding to a fault internal to the power system apparatus during the time it is restrained from operating in response to an external fault.
Differential relays have been used for many years to protect various power system apparatus, including generators, transformers, power lines and busses, among others. In the basic operation of a differential relay, incoming current to the power system apparatus for each phase (A, B and C) of the power signal is compared against the outgoing current from the power system apparatus. When the two currents are the same for each phase, the power system apparatus is operating properly. However, when an internal fault (internal to the power system apparatus) occurs, the two currents will be unequal, and by processing the two currents properly, an accurate determination of an internal fault can be made. The differential relay then operates (trips a circuit breaker) to remove the power system apparatus from the power system.
FIG. 1 shows a typical connection of a differential relay. A power system apparatus is shown in general block form at 10. Again, this could be several different kinds of apparatus, but in many cases is a bus line. The power system apparatus has incoming current I1 and outgoing current I2 on lines 11 and 13, respectively. Located on line 11 is a first current transformer (CT) 12, while a second current transformer 14 is located on line 13, for one phase of the three-phase power signal. The two CTs reduce the high current on the power lines. to an appropraite level for processing by a differential relay 16, which continuously compares the two currents I1 and I2. Similar circuit arrangements are provided for the other two phases of the power signal applied to the power apparatus 10.
In operation, differential relay 16 declares an internal fault condition if a selected percentage of a current value referred to as IOP (operating current value) is greater than a selected percentage of another current value referred to as IRT (restraining current value). IOP is defined as the absolute value of the sum of the vector (phasor) values of I1 and I2 as follows:
IOP=|{overscore (I)}1+{overscore (I)}2 |
IRT is defined as the sum of the absolute values of I1 and I2 as follows:
IRT=|{overscore (I)}1|+|{overscore (I)}2|
A differential relay is designed to distinguish between an internal fault and an external fault (external to the power apparatus). FIG. 2 shows a typical operating characteristic of a differential relay, in particular sloping line 18. Sloping line 18 differentiates the external fault condition region 20 from the internal fault region 22. The differential relay, as explained above, is designed to operate (trip) for an internal fault but not to trip (an action referred to as blocking) for a fault which is external to the power apparatus. The protected xe2x80x9czonexe2x80x9d which defines an internal fault extends between the two current transformers.
Differential relays typically perform well relative to external faults, as long as the individual CTs on the power line reproduce the primary current on the line accurately. When one Ct does not produce the primary current accurately, inaccurate, i.e. fictitious, current is presented to the differential relay which produces an inaccurate result. A typical cause of CT inaccuracy is the CT going into saturation when a fault occurs. When the CT for the I2 (outgoing) current saturates, for instance, the I2 value will be incorrect, and the differential relay may declare an internal fault condition, with a subsequent inappropriate raly operation (trip). This of course is quite undesirable.
Various solutions to the CT saturation problem (misoperation of the differential relay because of CT saturation) have been developed. In one solution, the second harmonic of the current signal is used. In another solution, the characteristic of the differential relay (the sloping line 18 in FIG. 2) is changed by increasing the slope to compensate for the CT saturation. These solutions, however, while providing improved operation in some situations, fail to present relay misoperation in other situations. Further, with respect to the first solution, the trip logic for the differential relay is completely blocked during the time when the second harmonic is above a set threshold. In another solution, the delay is blocked for a fixed period of time (e.g. 7 cycles) after the detection of an external fault condition.
The relay, however, cannot recognize an external fault which evolves to an internal fault or an internal fault which develops independently during the blocking time. This delays unnecessarily the recognition and response to an internal fault.
Still another solution involves the use of phase angle information of the respective currents. The phase angle information remains accurate even during CT saturation conditions. A directional relay, which is a common, well known protective element, uses the phase angle information to distinguish between external and internal faults. The combination of a differential relay with a directional relay can thus provide supervision of the operation of the differential relay to overcome some of the disadvantages of using a differential element by itself. However, such a combined arrangement slows down the operation of the differential relay, which can otherwise be quite fast in operation. This is due to the directional element. Further, the combination cannot detect high resistance faults for incoming and outgoing currents which are 180xc2x0 out of phase. Hence, it is desirable to have a differential relay which is capable of recognizing an external fault and preventing operation of the relay for that condition, but otherwise be able to operate properly, and as designed, at all times in response to an actual internal fault, even when an external fault evolves into an internal fault. It is also important for the differential relay to maintain its usual high speed operation, without being unnecessarily slowed by additional elements.
Disclosure of the Invention
Accordingly, the present invention is a differential relay for protecting a power system apparatus in a power system, the power system apparatus having a power line input and a power line output, comprising: means for detecting the power signal current present on the power line input and the power line output, respectively; means for recognizing a fault condition in the power system external to the power system apparatus; means for preventing a tripping signal in response to the external fault condition for a selected period of time; and means for recognizing an internal fault during said selected period of time, including the evolution of said external fault into an internal fault, and for permitting a tripping action during said selected period of time.